132
FXUS63 KLOT 082333
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
633 PM CDT Wed May 8 2024

.KEY MESSAGES...

- Showers and a few thunderstorms are expected this evening and
  tonight. Some thunderstorms may feature small hail and gusty
  winds, primarily outside of the Chicago metro area.

- Cooler weather arrives Thursday, with periodic showers
  Thursday and again Friday night into the weekend.

&&

.DISCUSSION...
Issued at 246 PM CDT Wed May 8 2024

Through Thursday night:

Today has been awfully pleasant for us so far with sunny skies and
temperatures having reached the middle 70s to near 80 degrees area-
wide. This is not the story across the rest of the Midwest, however.
Early this afternoon, we find two centers of surface low pressure in
the region: one subtle little system in northwest IA and a broad,
more prominent low marching across the Ozark Plateau. The former is
producing nothing more than light showers out west while the latter
is responsible for a swath of severe storms extending from western
MO into south-central Kentucky. These two systems will phase into
one over the next several hours before moving into central IL this
evening. This will bring showers and likely a handful of storms to
the area this evening through the better part of tomorrow.

We should begin to see isolated to widely scattered showers move
into the SW CWA early-mid evening and progress across the rest of
the area. With only a little bit of elevated instability squeezing
into the CWA, thunderstorms look rather unlikely with this first
little push any farther north than say the US Rt 24 corridor. More
widespread activity will work in not long after midnight along the
northern flank of the low as it passes through central IL. It looks
like the overnight period will be our best chance at seeing embedded
thunderstorms when several hundred joules of elevated CAPE look
to build over parts of the area. The greatest thunder potential
will exist in our western and southern CWA where we`ll find the
most appreciable instability with notably steeper low-mid level
lapse rates. The environment becomes less and less favorable
approaching the lake. While CAPE values certainly won`t stand
out tonight, most of it will be confined within the hail growth
zone and we will have a respectable amount of mid level shear to
work with, roughly 25-30 kt at 3-6 km. This means it`s not out
of the question to see some small hail develop within any
relatively stronger elevated cores. Some dry air in the low
levels and a ~30 kt LLJ could also promote some gusty winds with
any thunderstorm or heavier shower tonight.

The instability will wane to the east early tomorrow pretty much
driving the thunder potential out of here through the morning.
Scattered showers are expected to continue through much of the
afternoon before high pressure builds in and clears conditions up
during the evening. A tight pressure gradient on the backside of the
system will bring breezy winds to the area during the day. Between
the cloudy, rainy conditions and cold advection, tomorrow will be
noticeably cooler with highs forecast near 60 south of I-80 and
stuck in the 50s farther north.

Doom


Friday through Wednesday:

Shortwave ridging will produce stout mid-level warming through
the day Friday, though low-level moisture will remain sufficient
for a decent coverage of shallow cumulus during the afternoon.
On the heels of the ridge, a compact upper-level low will dig
southeast over the western Great Lakes Friday night into
Saturday. Earlier guidance had advertised a diurnally favorable
passage of the low directly over our area midday Saturday, but
has since supported a faster solution with the low passing to
the northeast around daybreak. Additionally, more favorable
thermodynamic profiles with anomalously cold mid-level
temperatures will remain to the northeast. Have therefore
continued to cap PoPs in the chance range with the expectation
that shower coverage will be no greater than scattered.

Northwest flow aloft will persist Sunday into early next week.
Another shortwave trough is progged to cross the area sometime
late Sunday afternoon through Sunday night. Low-level moisture
availability looks marginal at best for any widespread
precipitation as a sprawling surface ridge remains anchored in
the Deep South, but an earlier arrival of the trough could
support isolated to scattered storms in our forecast area during
the late afternoon and evening.

Beyond Sunday night, substantial differences in guidance
regarding the evolution of a building ridge across the eastern
seaboard and a central CONUS trough support leaving the NBM`s
slight chance and chance PoPs untouched from Monday through
Wednesday.

Kluber/Ogorek

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 633 PM CDT Wed May 8 2024

Key Aviation Impacts for the current TAF period:

- Rain moves in after midnight to Chicago terminals (just before
  midnight for KRFD) with MVFR cigs and vis

- Thunder cannot be ruled out but the probability is less than
  30 percent so it remains out of the current TAFs

- Breezy northeast winds Thursday morning as the system exits to
  the east as lingering showers hang around

VFR conditions continue as a BKN deck at 10000 feet AGL moves
in. Storms that are firing (currently) over western Iowa are
scheduled to gradually move eastward through the night and into
tomorrow morning. Light rain could arrive as early as 03Z, but
the strongest showers are not expected until after midnight at
Chicago terminals (just before midnight at KRFD). The stronger
showers are expected to bring cigs down to near IFR levels with
reduced visibility for MVFR conditions through Thursday
morning. With instability overnight expected to be pretty
meager, it provides the risk for lightning around 10 to 30
percent. While it cannot be completely ruled out, thunderstorms
were left out of the TAF at present and will be closely
monitored through the night. Strongest storms are expected to
move south and east of the area after 12Z. Based on the modeled
track, if there was an area where stronger storms might hold on
a little longer, it would be around KMDW and KGYY. But
confidence remained in leaving the TEMPOs ending at 12Z

As the center of the system moves east of the area, rain will
gradually transition to more showery activity through Thursday.
Winds will remain out of the northeast, but the potential for
gusts around or just over 20 knots is possible through the
morning and early afternoon, diminishing thereafter. Despite
remaining at MVFR conditions through the day, cigs should
gradually improve to around or over 2000 feet Thursday
afternoon, but confidence is low on the exact timing for cigs
improvement and when wind gusts diminish.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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